1. Field of the Invention
The present invention relates generally to photodetectors and more particularly to high speed metal-semiconductor-metal photodetectors with increased signal-to-noise ratios and data transmission rates for use in optoelectronic integrated circuits.
2. Description of the Related Art
High speed, metal-semiconductor-metal (MSM) photodetectors operating at speeds of 1 GHz and above have a wide range of uses in optoelectronic integrated circuits (OEICs). Of particular relevance is the application to optically based telecommunications systems. An illustration of an MSM photodetector representing the current state-of-the-art is shown in FIG. 1. This device consists of a generally rectangular array of interdigitated electrodes 10 deposited on an optically active layer of semiconductor material 20, typically GaAs or InGaAs. Usually the electrodes are chosen to form Schottky contacts at the electrode-semiconductor interface, but ohmic contacts have occasionally been used. Incident radiation is absorbed between the electrodes, which are connected such that every other electrode is at an opposite electrostatic potential. The electron-hole pairs produced in the semiconductor layer by the absorbed photons are transported to the electrodes by the strong electric fields produced by the bias voltage and then constitute a signal current.
The planar structure of the MSM photodetector results in lower capacitances for these devices than for any of the doped junction photodetectors (e.g., PN or PIN diodes). This improves the limitations on speed imposed by interaction of the device with the circuit into which it is embedded. The electrode finger width can be made extremely small to allow very fast transit-times of the photogenerated carriers, which is the other limiting factor in determining the overall system response time. Finally, the fabrication technology for these devices is fully compatible for that used in field-effect transistors, making these devices easy to integrate into monolithic high-speed fiber-optic receiver circuits.
In spite of these advantages, the rectangular, interdigitated electrode geometry currently used in MSM photodetectors has several disadvantages. First, the abrupt termination of the electrode fingers results in a non-ideal electrostatic distribution characterized by "crowding" of electric field lines in the regions near the ends of the electrodes. This can manifest itself in an increased capacitance and degraded dark current performance.
Secondly, the shape of this device is not well matched for interaction with optical signal sources. Most optical sources such as lasers and LEDs produce an intensity distribution that is circular in nature. The rectangular shape of the interdigitated, electrode geometry, MSM photodetectors is therefore not matched to these sources. This presents an unfavorable trade-off between signal-to-noise ratio and signal bandwidth. In order to fully detect an optical signal in this situation, the detector must be made with the length and width of the active area equal to the beam diameter, where the active area is defined as the area of the semiconductor layer exposed to incident radiation. This results in excess device area that is not used in the detection process but which still contributes to the device capacitance. The increased capacitance will result in a less than optimal signal bandwidth. Conversely, decreasing the active area of the detector to reduce capacitance, and thus increase bandwidth, results in a detector size smaller than that of the optical beam, degrading the signal-to-noise ratio.